Blood clots and the COVID-19 Vaccine

- 12 April 2021

The vaccine and blood clots – should you get your vaccine yet?

The incidence of blood clots following a dose of the Astra-Zeneca (AZ) vaccine is making news, with the Australian Technical Advisory Group on Immunisation (ATAGI) stating the preferred vaccine in people under 50 years of age is the Pfizer (Comirnaty) vaccine.

This post is not to tell you to get your AZ vaccine or not, but to give some guidance on the risk as we understand it so far.

Firstly, not all clots are the same. The clots potentially caused by the vaccine are thought to have a similar mechanism to a condition called Heparin Induced Thrombocytopaenia (HIT), and this new syndrome is being called many things, such as Vaccine Induced Prothrombotic Immune Thrombocytopaenia (VIPIT) which I will refer to it as in this post. Other than the fact that it appears to affect younger female patients, we don’t know any other risk factors. The female prevalence may be explained by the fact more females are immunised in Europe than males at present but this isn’t clear.

As of the 4th of April 2021, the European Medicines Agency (EMA) has recorded 34 million doses of the AZ vaccine given in Europe including the UK. There have been 169 cases of Cerebral Venous Sinus Thrombosis (CVST) and 53 cases of Splanchnic Vein Thrombosis. This means 222 cases in 34 million doses, and assuming all of these are due to the vaccine (which we can’t definitely say) this puts the risk at 6.52 VIPIT cases per million doses given.

The mortality rate in these patients in Europe is about 20%, but as with all emerging conditions it takes time to learn more and understand the process.

How does this compare with clots from other common medicines or lifestyle factors?

One of the most common risk assessments GPs make in regards to clots are the risk associated with the Combined Oral Contraceptive Pill, or ‘the pill’. It’s not a great comparison, because the pill is something people take daily, it’s been around for a long time, and the mechanism by which it causes clots is different to VIPIT and is well understood.

The baseline risk for developing a clot in women not taking the pill is about 4 per 10,000 women per year (0.04%). For women who take the pill, its 7-10 clots per 10,000 women per year (0.07-0.1%) or about 1 in 1000.

For pregnant and postpartum women (the time after birth) the risk is 20-30 per 10,000 women per year (0.2-0.3%), but this rate is higher in the 12 weeks post partum at 45-60 per 10,000 women per year (0.4-0.65%), and highest in the 2 days before and 1 day after giving birth at 300-400 per 10,000 women per year (3-4%).

To compare again, the risk at present of clots from the vaccine appear to be 0.00000652%. But I don’t think it’s fair to compare things like this. Because to understand the risk of something, you must compare it to the benefit provided by that intervention and make a decision based on that.

Risk vs benefit

In the UK, COVID-19 was and still is running rampant. Lots of people were being infected, many were dying, and many more were left considerably unwell. With current circulating levels of COVID in the UK, delaying the vaccine to 10 million people by 1 week would result in 16,000 more infections than could have been prevented.

If all these people were over 60, then we’d expect 1000 hospitalisations and 300 deaths.
If everyone was 40, we’d have about 16 deaths.

Other than death, hospitalisation and respiratory failure, people with COVID-19 also develop clots at a rate of about 16%. Those who are younger without medical comorbidities are more likely to suffer from ‘Long Covid’ also.

So in the UK their advice is for people under 30 to have a choice in which vaccine they receive. Because taking into account the number of cases, the number of lives and illness saved by vaccinating, the risk-benefit appears to break even at about age 30.

So why is it 50 in Australia? Because we don’t really have any community COVID to speak of. The chance of being unwell from COVID if you were to catch it remains the same, but the chance of contracting it in the first place is much lower here which is great for now.

This infographic is from the BBC and COVID rates reflect the situation there.

Short term risk vs long term risk

Again I don’t think it’s fair for people to decide if they will or won’t get the shot without weighing up all the risks and benefits in the long term. While case numbers are low in Australia, it has only been so because of extremely strict border control measures, lockdowns and aggressive testing. Now if this were to remain the case forever, then maybe we can just think about the risk of clot vs no clot.

But eventually the world will want to return to global travel. I know I want to go overseas and visit friends and family, and dare I say just have a holiday. This is unlikely to happen until a significant proportion of our population is vaccinated, and enough time passes to see how this provides us immunity as a group (or herd) and what happens when people who are vaccinated invariably do contract COVID, either the main strain or new strains.

So this is something you might consider when deciding if you go ahead or not.

The Pfizer Vaccine

While it is all well and good that ATAGI has recommended that the Pfizer vaccine is recommended for those under 50, the Department of Health has not said how people under 50 will receive their Pfizer Vaccine and from where.

At present the Pfizer vaccine is available for those in phase 1a, frontline health workers, border workers, and those living in aged care. However many people in this category are yet to receive their first vaccine, and many frontline healthcare workers had opted for the Astra-Zeneca vaccine in order to have some protection from COVID-19 earlier. So if we are comfortable with the uncertainty of when and how we would receive that vaccine, then we can wait.

Is this all a moot point?

At present we’re in phase 1b of the vaccine rollout. That means that all healthy, low risk individuals under 50 aren’t eligible for any vaccine anyway. This risk analysis appears to boil down to one simple thing (that is infinitely difficult to measure with certainty) – are you more likely to die if you receive the vaccine or not. Almost anyone who qualifies for 1b for because of pre-existing medical conditions, COVID-19 will cause you much more trouble than the vaccine.

If you are otherwise perfectly healthy and are at risk because of your profession, then you need to consider all the info above, and consider new info as it comes to light and decide do you want to be immunised now, or wait until another vaccine is on offer. If you aren’t eligible for 1b yet anyway, then watch this space.

A final word on assessing risk

Deciding how a risk applies to ourselves or others is incredibly difficult. We’re not good at looking at things over the long term as individuals. If we were, we wouldn’t need to have compulsory super, no one would smoke, skin cancer would disappear and life would be very simple.

Risk is multifactorial, some risk factors compound, and others are minimised with multiple variables all at play. In essence though it is an extremely personal decision.

I hope that the government, our health bodies, and the media all do their due diligence in conveying information in an easy to understand and transparent fashion.

I (Dr Daniel Chanisheff) have had the Astra-Zeneca vaccine before the ATAGI guideline was released. Being a healthcare worker, I am in phase 1b, and having met patients who have had COVID themselves, patients who have family members they have lost overseas, I am acutely aware that this is not an Australa only issue. If I was due to receive the first dose of the vaccine today, I would probably do it. I understand there are bigger risks in my life than clots from this vaccine, I see what can happen if COVID infects us, and I want to be able to travel the world again one day. But I won’t tell any patient how to make their decision regarding the vaccine. I hope I can inform you to the best of my ability, and hope this makes your choice easier, even if only slightly.

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